Aliphatic isocyanates having uretdione groups are valuable raw materials for, inter alia, producing polyurethane coatings. Products based on optionally branched, linear-aliphatic diisocyanates have a particularly low viscosity. Products based on cycloaliphatic diisocyanates are generally highly viscous to solid substances which can be used as elimination product-free, internally blocked crosslinkers in coating systems. A summary is provided in J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200.
Tris(dialkylamino)phosphines (DE-A 3 030 513), if appropriate in combination with cocatalysts (DE-A 3 437 635), display good selectivity for the formation of uretdione groups (uretdione selectivity). However, the serious problem of the high carcinogenic potential of their phosphorus oxides, e.g. hexamethylphosphoramide, stands in the way of their industrial use.
DE-A 3 739 549 discloses the catalytic NCO dimerization using 4-dialkylamino-pyridines, e.g. 4-dimethylaminopyridine (DMAP), although uretdione formation proceeds selectively only in the case of specific cycloaliphatic isocyanates such as isophorone diisocyanate (IPDI). Linear-aliphatic isocyanates such as hexamethylene diisocyanate (HDI) and also branched, linear-aliphatic isocyanates such as trimethylhexane diisocyanate (TMDI) and methylpentane diisocyanate (MPDI) give mainly strongly coloured, heterogeneous reaction products when DMAP and related compounds are used.
DE-A 1 670 720 discloses the preparation of aliphatic polyisocyanates having uretdione groups, with tertiary phosphines having at least one aliphatic substituent and also boron trifluoride and its adducts being used as catalysts. The uretdione selectivity here is temperature-dependent, and appreciable amounts of viscosity-increasing isocyanate trimers (isocyanurates and iminooxadiazinediones) and, particularly at temperatures of >80° C., other undesirable by-products such as carbodiimides or uretonimines are always formed.
One way of increasing the uretdione selectivity and decreasing the by-product formation is the use of specific, bulky phosphines having P-bonded cycloalkyl groups (EP-A 1 422 223, unpublished German Patent Application DE 10354544). The fact that the uretdione selectivity for a given phosphine catalyst can be improved by addition of additives or solvents has been examined in JP 11228524. According to this, nonaromatic solvents having a Hildebrand solubility parameter of greater than 7 cal1/2 cm−3/2 are suitable for preparing polyisocyanates rich in uretdione groups in the presence of phosphine catalysts. However, as has been able to be shown in the examples of the present patent application, this is not always reliably the case (cf. chloroform; Hildebrand parameter 9.3 cal1/2 cm−3/2, compared to N-methylpyrrolidone, NMP; Hildebrand parameter 11.3 cal1/2 cm−3/2). Consequently, a person skilled in the art cannot derive any general teaching as to which solvents are reliably suitable for increasing the uretdione selectivity and reducing by-product formation and which are not from JP 11228524.
In addition, numerous publications disclose carrying out isocyanate oligomerizations in the presence of solvents in general terms without, however, naming individual compounds and/or specifically indicating compounds which lead to an improvement in the uretdione selectivity and a reduction in by-product formation.
It has now been found that the paired use of phosphines and organic carbonates and/or organic nitriles as catalyst system has a particularly advantageous effect on the selectivity of uretdione formation.